Major CSS revamp

- Get rid of Bootstrap (except we still rely on some JS)
- Use BEM class names
- Rework variables/theme config
- Fix various bugs, including some on mobile

The CSS is still not ideal – it needs to be cleaned up some more. But
that can be a focus for after beta.

less/lib/Alert.less

@@ -0,0 +1,44 @@
+.Alert {
+  padding: 12px 16px;
+  border-radius: @border-radius;
+  background: @alert-bg;
+  line-height: 1.5;
+
+  &, a, a:hover {
+    color: @alert-color;
+  }
+}
+.Alert--error {
+  background: @alert-error-bg;
+
+  &, a, a:hover, button, button:hover {
+    color: @alert-error-color;
+  }
+}
+.Alert-controls {
+  list-style-type: none;
+  padding: 0;
+  margin: 0 -8px 0 8px;
+  display: inline-block;
+
+  > li {
+    display: inline-block;
+    margin: 0 5px;
+
+    > a {
+      text-transform: uppercase;
+      font-size: 12px;
+      font-weight: bold;
+
+      &.disabled {
+        cursor: default;
+        text-decoration: none;
+        opacity: 0.5;
+      }
+    }
+
+    > .Button {
+      margin: -10px;
+    }
+  }
+}